For non-standard tags, there are two pieces to setting it up as a custom tag:
1) Figure out what the tag code is. (You can use Tag Inspector to find this info. For MP3s, it will be the TXXX/tagname value under Tag Code. For other formats it won't have TXXX/.) Use this code to create a new custom tag in Define New Tags. The value for your file format needs to match what you saw in Tag Inspector, except leave off the TXXX/ prefix for id3.
2) Link the new tag you created to a Custom Tag slot in Tags(1) and give it an appropriate name. When you save this setting, MusicBee will scan in the values from the new tag and it will show up in the Tag Editor on the second page of tags.
If all files have been scanned and you're not seeing values for this field in the Tag Editor, but you do see it in Tag Inspector, then either there's a mismatch between the tag code and the value you gave MusicbBee in Define New Tags, or you didn't link the Custom Tag slot correctly in Tags (1).